The performance of a centrifugal fan with enlarged impeller

The performance of a centrifugal fan with enlarged impeller
The influence of enlarged impeller in unchanged volute on G4-73 type centrifugal fan performance is investigated in this paper. Comparisons are conducted between the fan with original impeller and two larger impellers with the increments in impeller outlet diameter of 5% and 10% respectively in the numerical and experimental investigations. The internal characteristics are obtained by the numerical simulation, which indicate there is more volute loss in the fan with larger impeller. Experiment results show that the flow rate, total pressure rise, shaft power and sound pressure level have increased, while the efficiency have decreased when the fan operates with larger impeller. Variation equations on the performance of the operation points for the fan with enlarged impellers are suggested. Comparisons between experiment results and the trimming laws show that the trimming laws for usual situation can predict the performance of the enlarged fan impeller with less error for higher flow rate, although the situation of application is not in agreement. The noise frequency analysis shows that higher noise level with the larger impeller fan is caused by the reduced impeller–volute gap.
An implicit, time-accurate 3D Reynolds-averaged Navier-Stokes (RANS) solver is used to simulate the rotating stall phenomenon in a plastic centrifugal fan. The goal of the present work is to shed light on the flow field and particularly the aerodynamic noise at different stall conditions. Aerodynamic characteristics, frequency domain characteristics, and the contours of sound power level under two different stall conditions are discussed in this paper. The results show that, with the decrease of valve opening, the amplitude of full pressure and flow fluctuations tends to be larger and the stall frequency remains the same. The flow field analysis indicates that the area occupied by stall cells expands with the decrease of flow rate. The noise calculation based on the simulation underlines the role of vortex noise after the occurrence of rotating stall, showing that the high noise area rotates along with the stall cell in the circumferential direction.
As the power source of the air and gas system in the thermal power plant, the operation status of the centrifugal fan is directly related to the safe and economic operation of the power plant. Rotating stall in the centrifugal fan is a local instabilities phenomenon in which one or more cells propagate along the blade row in the circumferential direction. The nonuniform flow, the so-called stall cell, rotates as a fraction of the shaft speed, typically between 20% and 70%. This running mode is responsible for strong vibrations which could damage the blades [1]. Meanwhile, it will increase the aerodynamic noise.

In order to reveal the generation mechanism of rotating stall, lots of models and theories have been proposed since the 1960s. Especially, experimental methods were widely used to illustrate the characteristics of internal flow field during stall. Lennemann and Howard discussed the causes of stall cells in low flow rate condition through the hydrogen bubble flow visualization method [2]. Lucius and Brenner experimentally studied the speed variation of a centrifugal pump in rotating stall stage [3]. For the centrifugal turbomachine, multiple factors can affect the characteristics of stall. Vaneless diffuser, for example, has significant influence on stall. Hasmatuchi et al. experimentally investigated the effect of blowing technology on the flow field of a centrifugal pump under rotating stall [4]. Rodgers conducted an experimental research on rotating stall in a centrifugal compressor with a vaneless diffuser and found that the stall margin can be improved through adjusting the expansion pressure factor [5]. Abidogun carried out an experiment to investigate the influence of vaneless diffuser on the stall characteristics. The results showed that increasing the length of diffuser can improve the rotating speed of stall, and the change of width showed no effect on stall [6].

Further efforts were made to study the stall inception in order to avoid the occurrence or minimize the effect of stall. As well accepted, two types of stall inception proposed by Camp and Day modal wave inception and spike inception were investigated experimentally [7]. Leinhos et al. studied development process of stall inception under instantaneous inflow distortion in an axial compressor [8].
With the rapid development of computer technology, numerical simulation has become an important method for flow field research of turbomachine under rotating stall conditions. Gourdain et al. investigated the ability of an unsteady flow solver to simulate the rotating stall phenomenon in an axial compressor and found that it was necessary to take the whole geometry into consideration to correctly predict the stall frequency [1]. Choi et al. investigated the effects of fan speed on rotating stall inception; the results showed that, at 60% speed (subsonic), tip leakage flow spillage occurred successively in the trailing blades of the mis-staggered blades [9]. Zhang et al. numerically studied the stall inception in a centrifugal fan, and the results showed that the stall inception experienced probably 50 rotor cycles developing into a stall group. The inception showed significant modal waveform. The importance of volute for generation of stall inception was illustrated through flow field analysis [10].

Aerodynamic noise is mainly caused by vortex and flow separation. So the unsteady behavior of rotating stall may have an influence on the noise of centrifugal fan. In capturing the physical mechanism of the fan noise associated with rotating stall, the primary work is to characterize the noise. During the 1960s, the interaction between noise and turbulence was discussed by Powell, and the vortex sound theory was proposed to explain the generation of acoustic sound. Then, Lighthill made a breakthrough in aerodynamic noise theory research by proposing the acoustic analogy [11]. Based on these works, Díaz et al. put forward a prediction of the tonal noise generation in an axial flow fan, and the noise level in the plastic centrifugal blower far-field region was estimated by means of acoustic analogy [12]. Scheit et al. analyzed the far-field noise in a metal centrifugal fanwith an acoustic analogy method and presented design guidelines to optimize the radiated noise of the impeller [13]. The global control of subsonic axial fan at the blade passing frequency was also discussed by Gérard et al. [14]. He aimed at cancelling the tonal noise by using a single loudspeaker in front of the fan with a single-input-single-output adaptive feedforward controller. According to Ouyang et al.’s work, the far-field noise generated by cross-flow fan with different impellers was measured and it showed the great influence of blade angles on the inflow pattern [15]. Based on the previous research, a new method to predict the fan noise and performance is developed by Lee et al., and through an acoustic analogy, the acoustic pressures from the unsteady force fluctuations of the blades are obtained [16].

In summary, a wide range of flow characteristics on rotating stall in compressor have been investigated and the researches concentrated on stall inception. The present work focuses on two aspects: simulation of the rotating stall phenomenon with a 3D flow solver and seeking the deep physical mechanism of this instability in a centrifugal fan. The numerical method is presented with the model and the particular boundary conditions are used. Results from the whole geometry simulation are then analyzed. In the first part, aerodynamic characteristics and frequency domain characteristics of the centrifugal fan under different stall conditions are analyzed. In the second part, the velocity vector field distributions in the centrifugal fan are discussed. Finally, noise characteristics of the centrifugal fan under different stall conditions are studied. And the noise characteristics during the circumferential propagation of stall cells are also discussed.

2. Centrifugal Fan Description
The configuration of range hood centrifugal fan studied in this work is shown in Figure 1. It is composed of current collector, impeller with 12 airfoil blades, and the volute. The inlet and outlet diameter of the impeller are 568 mm and 800 mm, respectively. The inlet and outlet width of impeller are 271 mm and 200 mm, respectively. The nominal rotation speed is 1450 rpm. The volute tongue gap is 1% of the impeller outlet diameter. The width of the rectangular volute is 520 mm, and a simple antivortex ring is set inside the volute to reduce the generation of vortex. At the design operating point, the volume flow is 6.32 m3/s and the full pressure is 1870 Pa.
As shown in Figure 8(b), under the combining influence of both stall cell and volute tongue, the high noise area is gradually elongated. Due to the propagation of stall cell, it gradually gets away from the area of volute tongue, resulting in weakening the superimposing effect. As time goes by, the high noise area in Figure 8© gets further elongated with a trend of separation and the sound power level of high noise areas decreases. In Figure 8(d), the high noise areas corresponding to the vortex noise and volute tongue noise basically separate. And the sound power level corresponding to volute tongue greatly declines.

It can be drawn from Figure 8 that while the impeller passes three passages along clockwise direction, the high noise area passes two impeller passages along the clockwise direction. It indicates that, in the absolute coordinate reference system, the high noise area occupying about three impeller passages rotates in the same direction with impeller under rotating stall. It also has the same speed with stall cells, while in the relative coordinate reference system, high noise area spreads in the opposite direction of the rotation of the impeller.

Through the analysis above, there are two major sources of noise in a centrifugal fan under rotating stall, namely, the vortex noise caused by stall and the volute tongue noise caused by the rotation of impeller. When the stall cell spreads to the volute tongue, due to the superimposing effect of vortex noise and volute tongue noise, the sound power level is the highest and the high noise area is the largest. While the stall cell is away from the volute tongue, the corresponding high noise areas separate gradually. Along with that, the sound power level decreases and the high noise area becomes smaller. Therefore, the aerodynamic noise of the centrifugal fan under rotating stall changes periodically over time, and the fluctuation period is the same with the rotating period of the stall cell.
The authors declare that they have no financial or personal relationships with other people or organizations that can inappropriately influence their work; there is no professional or other personal interests of any nature or kind in any product, service, and/or company that could be construed as influencing the position presented in, or the review of, this paper.